Serpens, re 281: I think the difference is that rocks ejected from Earth are being caught by the gravitational well of Mars much easier and subsequently impacting than it is the case of Martian material shooting into the inner solar system crossing Earth orbit but just missing. Martian material probably more rely on "direct hits" to end up on Earth while Earth material don't need to be aimed that precise to impact Mars.

Barsoomer

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Reply: 284

Posted: January 30, 2012 1:09 AM

Serpens> ... throw a rock up from the empire state building and throw one down and see which one goes further.

That is a false analogy. Think of it this way: In order to get from Mars orbit to Earth orbit, one must lose orbital energy. To go in the other direction, one needs to gain orbital energy. By the Law of Conservation of Energy, the energy delta must be the same in both cases.

Your analogy is false because if a rock is ejected from Mars and enters a solar orbit, it does not automatically fall inward toward the Sun. The centrifugal force it has in Mars orbit exactly counter-balances the inward pull of the Sun. Some of the orbital velocity must be cancelled in order for the rock to enter a lower orbit. Surely you know this?

Barsoomer

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Reply: 285

Posted: January 30, 2012 1:16 AM

Just to clarify: in reply 284 by Earth orbit and Mars orbit, I mean SOLAR orbits at the distances of Earth and Mars, respectively, from the Sun. I don't mean orbits around Earth and Mars.

Serpens

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Reply: 286

Posted: January 30, 2012 3:32 AM

Barsoomer. The laws of physics have been suspended. The delta is + and -.

Barsoomer

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Reply: 287

Posted: January 30, 2012 11:19 AM

Serpens, think of it this way: If a spaceship needed to transfer from a Mars-distance solar orbit to an Earth-distance solar orbit, the amount of propellant required should be comparable to a transfer in the opposite direction.

There could be other reasons why the flux of mars meteorites on Earth might be greater than that of Earth meteorites on Mars, but the point is that there should be plenty in any case---enough to transfer biological materials. Your argument seems to require that the transfer is zero or negligible.

Serpens

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Reply: 288

Posted: January 30, 2012 5:41 PM

Barsoomer. Basic physics. Escape velocity is (2GM/r)^0.5. So on Earth an impact would need to give a piece of ejecta a kick off velocity of 11.2 km/s. On mars a mere 5 km/s is required. Inference. A lot more ejeca from an impact on Mars would reach escape velocity. Once escape velocity is achieved the dominant influence is the suns gravity which provides an inward acceleration. Consider this - from earth orbit a kick off velocity of around 42 km/s would be required to leave the solar system completely (34 k/s for mars) . So if a piece of ejecta from a martian impact reaches Martian escape velocity the suns gravitational field would impart an inwards acceleration and most would eventially move inwards regardless of original vector. Whether the martian ejecta impacts Earth would be a function of Earth's position in its orbit. But to reach Mars' orbit ejecta from an Earth impact requires a extra couple of km/s and in additon only only a small population would have the appropriate vector. Mars is a smaller gravitational well than Earth (smaller target) and the orbit is much larger. So can you understand why the probability of ejecta from an Earth impact reaching Mars is tiny compared to the probability of ejecta from a Martian impact reaching Earth?

Barsoomer

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Reply: 289

Posted: January 30, 2012 8:17 PM

Serpens, I don't disagree with most of your physics analysis, but I do disagree with this: "So if a piece of ejecta from a martian impact reaches Martian escape velocity the suns gravitational field would impart an inwards acceleration and most would eventially move inwards regardless of original vector."

If a piece of ejecta from a martian impact reaches Martian escape velocity, it leaves Mars but then enters a solar orbit similar to that of Mars. At that point, it is no more inclined to move inward than Mars itself.

In fact, ejecta from both Earth and Mars that enter a solar orbit will be perturbed by subsequent near encounters with Earth and Mars and will follow chaotic trajectories where they could end up anywhere.

I will concede that because of Earth's larger gravity well, more debris from Mars is likely to end up on Earth than vice versa. But even if the difference is two or three orders of magnitude, that still is an enormous amount of material, and only ONE viable rock falling into the ocean is enough to transfer biological material.

So to my mind, the assertion that biological material is unlikely to be transported from Earth to Mars is an extraordinary claim, and I would need to feel confident that someone has reliably done the calculations before I would be convinced of it.

Serpens

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Reply: 290

Posted: January 30, 2012 9:33 PM

Barsoomer. Think vectors.

Kevin

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Reply: 291

Posted: January 31, 2012 4:59 AM

Don't forget the ejecta could get caught by the gravity of our Moon just as easily as falling toward the Sun and either crash into it or experience a sling shot to who knows where.

John Henry Dough

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Reply: 292

Posted: February 4, 2012 11:26 PM

Serpens is correct,,,,,the gravity well is inward toward the Sun not outward,,,unless Jupiter,,,but again vectors.

Barsoomer

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Reply: 293

Posted: February 5, 2012 12:07 AM

JHD, ask yourself why Mars does not fall into the Sun?

Serpens

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Reply: 294

Posted: February 5, 2012 7:25 AM

Barsoomer. If something is in Earth orbit and gets delta V applied in the right direction it will move to a higher orbit. In the 'wrong' direction it will head in system. Dependong on the vector you could maybe get a lander or satellite to Mars, or hey, maybe even with slingshots leave the solar system altogether. You need the slingshots because the sun will continue to provide a deceleration inwards. Impact Mars with a big enough rock and hey, the orbital distance of the planet will change. So if a bit of ejecta has an escape velocity with vector in system it will contue to accelerate. If the vector is out system it will decelerate and then start in system.

Barsoomer

Posts: 344

Reply: 295

Posted: February 5, 2012 1:05 PM

Debris ejected from a planet starts out by inheriting the orbital velocity of the planet. Any residual velocity beyond the escape velocity is added to the orbital velocity. The residual velocity is likely to be small compared to the orbital velocity. The addition is, yes, vector addition. The direction of the residual velocity is random, so the resulting velocity has a direction that is either slightly inwards or slightly outwards, the probability of each being roughly 50%.

At this point, the debris is subject to random gravitational perturbations, probably mainly from the planet it just left, often called the "slingshot" effect. Again the direction of the perturbations would be about 50% outward and 50% inward. The probability that it moves inward is about the same as the probability that it moves outward.

Over the long-term, debris will perform a random walk through the solar system until it either enters a stable orbit somewhere, or falls to a planet, or is ejected from the solar system.

It is conceivable to be me that if one performed a detailed analysis of such random walks, one might then find a bias in one direction or another, but I would want to see calculations that confirmed that, not hand-waving arguments based on naive intuitions. In any case, even if more travels inwards than outwards, my original point only requires that a substantial fraction should travel outwards.

Serpens

Posts: xxx

Reply: 296

Posted: February 5, 2012 6:10 PM

Barsoomer. I have to admit that you really have me scratching my head on this one. Random gravitational pertubations correlated to the slingshot effect? The slingshot effect can be used to slow an object or speed it up depending on whether it passes in front or behind the major body.

Mars' mean orbital velocity is around 24 k/s. Escape velocity is 5 k/s. Now some impact ejecta will exceed that speed and since this thread started with Earth ejecta reaching Mars meaning that the impact kick off velocity for some material must be much greater than the Earth escape velocity of 11 k/s. So we could expect some martian ejecta to have a residual velocity (initial less mars escape velocity) of well over 6 k/s. So take into account as you say initial (orbital) velocity, impact kick off velocity, Mars gravitational acceleration and the suns gravitational acceleration and various scenarios for where the impact occurred on Mars and you can get an idea of the potential orbital paths of such debris.

Perhaps you may like to get hold of a book on orbital mechanics because the only hand-waving arguments based on naive intuitions are foi=und in your post.

Barsoomer

Posts: 344

Reply: 297

Posted: February 5, 2012 6:43 PM

> ...Earth ejecta reaching Mars meaning that the impact kick off velocity for some material must be much greater than the Earth escape velocity of 11 k/s.

Not so. It initially only needs to escape Earth's influence and then subsequent slingshot gravitational influences of both the Earth and the moon may eventually impart the required velocity to reach Mars or elsewhere.

You are also ignoring the point that only a fraction (indeed only one viable one) of the rocks need to reach Mars during a warm wet period in order to "infect" Mars with terrestrial life.

Serpens

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Reply: 298

Posted: February 5, 2012 8:22 PM

Barsoomer. If an impact on Earth would provide an escape velocity of 11 k/s then it can be assumed that an impact on Mars would produce the same kick off velocity. With a kick off velocities greater than 5 k/s and the material would escape from Mars and out into the dark and Mars orbotal velocity will not dominate the vector calculation.. But for an Earth impact anything less than 11 k/s will not escape so a heck of a lot more Martian materiel would escape Mars.

It is plausible that the moon could provide a slingshot effect providing an increased velocity outwards from the Earth Luna system if the ejecta is on the right trajectory. Equal probability that it would reduce the velocity. But do a few calculations on the back of an envelope (the back of a fag packet no longer being acceptable in our risk adverse society) and see just what a low probability event you are proposing.

Not ignoring the point re panspermia colonising Mars. Just saying that the probability (being the product of a number of low probability events) is extremely low. Also making the point that the number of Martian meteorites found on Earth do not indicate that a similar amount of material may reach mars.

Barsoomer

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Reply: 299

Posted: February 6, 2012 12:05 AM

Suppose the probability that one rock ejected from Earth will reach Mars = (1/N) where N is very large. That is a tiny probability. But suppose N^2 rocks are ejected from Earth. Then the probability that at least one rock reaches Mars is

1 - [(1-1/N)^N]^N

Now the limit of (1-1/N)^N as N tends to infinity equals 1/e, where e is the base of natural logarithms. So the whole expression tends to 1 -(1/e)^N, which tends to 1.

Of course, it may be N^(1/2) rather than N^2 or something else entirely. Who knows?
That is why the numbers matter. One needs to do a simulation to determine what the actual probability is. My guesstimate is that it is large; yours seems to be that it is small, but the magnitude of the real figure is not obvious.

Now, 12 meteorites, and counting, have been identified as coming from Mars. Given that the overwhelming majority of meteorites fall where they will never be seen, let alone collected, that implies the total number of Mars meteorites that have fallen to Earth must be enormous.

One point that you have not raised, but gives me pause, is that so far no meteorites have been identified as coming from Venus (but apparently one from Mercury has been suggested). I'm hoping that when we get back to the moon, it will be heaven for meteorite collectors.

Serpens

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Reply: 300

Posted: February 6, 2012 4:19 AM

Barsoomer, Oh please! The basic function would encompass the product of the following probabilities:

a. That a piece of ejecta from an impact on Earth has a kick off velocity exceeding escape velocity (11 km / sec;

b. The ejecta has sufficient velocity additional to escape velocity to offset the suns gravitational acceleration and reach Mars orbit;

c. That the vector of the ejecta is in the plane of the ecliptic;

d. That Mars and ejecta will be in the same place when the ejecta crosses Mars orbit;

e. That the ejecta contains live biological material that survived the impact heat and shock necessary to impart the huge impact kick off velocity required;

f. That the biological material in/on the ejecta could survive the time in space;

f. The ejecta lands in the narrow historical window where liquid water was present on Mars;

g. The environmental conditions are such that the organism can survive.

I can think of more but there would already be too many zeros after the decimal point.

It is easier to accept that the much lower kick off velocity for martian ejecta (escape at 5 km/sec)is far more likely and since the gravitational gradient for such material is than towards the sun it will all eventually infall, some crossing Earths orbit.

A heck of a lot more material from Mars would reach Earth than vice versa.